1. Field of the Invention
The present invention relates to mesogenic materials, and more particularly to a new class of mesogenic materials having smectic C* and smectic A* phases and exhibiting ferro-electric and electroclinic properties. The invention also relates to mixtures of these compounds.
2. Description of the Related Art
Liquid crystals used in display devices typically are ferroelectric liquid crystals or electroclinic liquid crystals. Typically, a mixture of several mesogenic (i.e., possessing at least one liquid crystal phase) materials are used in a given device. By carefully selecting components and ratios in a mixture of liquid crystals, optimized properties may be obtained that could not be achieved with a single liquid crystal.
A. Ferroelectric Liquid Crystals
Ferroelectric liquid crystal materials (FLC) have a permanent electric polarization in the absence of an applied electric field (analogous to the permanent magnetic polarization of ferromagnetic materials in the absence of an applied magnetic field). These ferroelectric materials are useful for display devices. In particular, these materials are useful for surface-stabilized ferroelectric liquid crystal (SSFLC) display devices.
Desirable properties for a FLC, or for a mixture of FLCs in an SSFLC device include: (1) displaying the ferroelectric smectic C* phase over a broad range of temperatures, (2) possessing a low melting temperature so the ferroclectric phase is stable to sub-ambient temperatures (i.e., below about 25.degree. C.), (3) possessing a large tilt angle (&gt;20.degree.), and (4) having a fast electro-optical response time so that framing rates of 1 kHz or more are achievable.
Existing FLCs are unsatisfactory to one degree or another.
B. Electroclinic Liquid Crystals
In the presence of an applied electric field parallel to the smectic planes, molecules of an electroclinic liquid crystal will tilt with respect to the plane of this applied electric field. The magnitude of the tilt is proportional to the of the applied field. The proportionality constant is generally referred to as the electroclinic coefficient, and is represented as d.theta./dE. Because of this proportionality between field and tilt angle, electroclinic materials provide a gray-scale capability that ferroelectric materials do not provide.
These electroclinic materials are also useful for a range of applications, including optical and image processing, optical correlators and interconnects, real time holography, and neural network circuit elements. Desirable properties for an electroclinic liquid crystal, or for a mixture of electrocilnic liquid crystals in a device include: (1) displaying the electroclinic smectic A* phase over a broad range of temperatures, (2) possessing a low melting temperature so the electroclinic phase is stable to sub-ambient temperatures (i.e., below about 25.degree. C.), (3) possessing a large induced tilt angle (&gt;20.degree.) for maximum contrast, (4) having this large induced tilt angle over a broad temperature range, and (5) having a fast electro-optical response time so that framing rates of 1 kHz or more are achievable.
Existing electroclinic liquid crystals are unsatisfactory to one degree or another. See U.S. Pat. No. 5,168,381, issued Dec. 1, 1992 to Walba, U.S. Pat. No. 5,116,527, issued May 26, 1992 to Coates et al., and U.K. Patent Application No. 9111025.4, by Graham et al. and published Dec. 4, 1991.